STM-based molecular detection of “catch-and-release” of protons for bipyridine bound to phenylene–ethynylene thiol

Synthesis and Surface Assembly of Ruthenium Bipyridine Complexes

Disulfide-functionalized bipyridine and the corresponding thiol derivative were prepared starting from xanthogenate-functionalized bipyridine. The reduction of the xanthogenate by hydrazine led to the formation of a mixture of thiol and disulfide-functionalized bipyridines that could be separated by size exclusion chromatography. The chelating properties of the bipyridine units were used to prepare fluorescent heteroleptic ruthenium complexes, whereas the thiol or disulfide group on the other end of the molecule was used to anchor the individual ligands as well as the formed complexes to solid substrates. Scanning tunnelling microscopy was used to visualize the assembly of these species on a flat gold(111) substrate.

Surface Potential Switching by Metal Ion Complexation/Decomplexation Using Bipyridinethiolate Monolayers on Gold

Surface potential switching on gold(111) surfaces is induced by complexation/decomplexation reactions of a bipyridine (BP) derivative and palladium(II) chloride, as observed by Kelvin probe force microscopy (KFM). On the basis of the theoretical predictions, a 4-(5-phenylethynyl-2,2'-bipyridine-5'-yl-ethynyl)benzenethiol (PhBP) derivative was synthesized and used as an active monolayer to catch transition metal ions. By using the microcontact printing (CP) technique, micron-size patterned PhBP monolayers, which act as effective hosts to coordinate palladium(II) chloride, were prepared on gold(111) surfaces. The KFM signal decreases by complexation of the Pd(II) chloride in PhBP monolayers and is recovered by removal of Pd ions using an ethylenediamine solution, as confirmed by X-ray photoelectron spectroscopy. This process is reversible, indicating that the surface potential switching is realized by complexation/decomplexation of Pd(II). A CP PhBP monolayer, when it detects the target palladium ion, shows sensitivity for the picomolar level detection judged from surface potential changes in KFM measurements. The dipole moment estimated by the surface potentials is much smaller than the calculated value, indicating that mechanisms for the reduction of the surface dipole moment exist in real monolayers prepared by the CP method.